antiMarkovnikov

Anti-Markovnikov refers to a set of regioselective additions to alkenes where the added group attaches to the less substituted carbon of the double bond, opposite to the traditional Markovnikov outcome. The term covers several distinct reaction types that produce anti-Markovnikov products under specific conditions or with particular reagents.

A well-known example is the hydrohalogenation of alkenes with hydrogen bromide in the presence of peroxides. The peroxide effect enables a radical mechanism in which Br• adds to the less substituted carbon to form a more stable radical, which then abstracts a hydrogen atom from HBr to give the anti-Markovnikov bromoalkane and regenerate Br•. Thus bromine ends up on the less substituted carbon, and hydrogen on the more substituted carbon.

Another principal case is hydroboration-oxidation. Here, borane adds to the less substituted carbon of the alkene, delivering hydrogen to the more substituted carbon. After oxidation, the boron is replaced by a hydroxyl group, yielding an alcohol with the OH on the less substituted carbon. This process provides anti-Markovnikov hydration of terminal alkenes, often producing primary alcohols from simple alkenes.

In general, anti-Markovnikov selectivity arises from alternative mechanisms or reagents that favor attachment to the terminal, less substituted carbon. The concept is named in reference to Vladimir Markovnikov, whose rule describes the opposite regioselectivity under standard, non-radical conditions. Anti-Markovnikov methodologies are valuable tools in organic synthesis for constructing primary alcohols and other anti-Markovnikov products.